Constraints on the Parental Melts of Enriched Shergottites from Image Analysis and High Pressure Experiments

Martian basalts can be classified in at least two geochemically different families: enriched and depleted shergottites. Enriched shergottites are characterized by higher incompatible element concentrations and initial 87Sr/86Sr and lower initial 143Nd/144Nd and 176Hf/177Hf than depleted shergottites. It is now generally admitted that shergottites result from the melting of at least two distinct mantle reservoirs. Some of the olivine-phyric shergottites (either depleted or enriched), the most magnesian Martian basalts, could represent primitive melts, which are of considerable interest to constrain mantle sources. Two depleted olivine-phyric shergottites, Yamato 980459 and Nothwest Africa (NWA) 5789, are in equilibrium with their most magnesian olivine and their bulk rock compositions are inferred to represent primitive melts. Larkman Nunatak 06319 and NWA 1068, the most magnesian enriched basalts, have bulk Mg# that are too high to be in equilibrium with their olivine megacryst cores. Parental melt compositions have been estimated by subtracting the most magnesian olivine from the bulk rock composition, assuming that olivine megacrysts have partially accumulated.
However, because this technique does not account for the actual petrography of these meteorites, we used image analysis to study these rocks history, reconstruct their parent magma and understand the nature of oli-vine megacrysts.
Our image analysis results are supported by a series of high-pressure experiments performed on LAR 06319 bulk rock composition to test if it could represent a primitive melt, and understand its crystallization history.